CHAPTER IV ESTABLISHMENT AND CULTIVATION OF SPECIALIZED CROPS FOR HAY

Establishing pasture grasses

Perennial pastures are installed for two to five years or more, and shortcomings at establishment cannot be rectified during the life of the crop. It is essential to obtain an even stand across the field, and all operations should be designed to assure this. The small-seeded grasses specially grown as pasture and fodder differ, as crops, from the cereals in that they are mostly of very recent domestication. Indeed some of the tropical species are barely domesticated. Temperate pasture grasses have, at best, been in cultivation for a few hundred years, while most of the tropical species were first grown on a field scale this century. Pasture grass seeds are very small (see Table 3) and their sowing requires care and attention to detail; the few days of establishment are the most crucial for a new pasture.

Some tropical grasses can be vegetatively propagated (usually large or stoloniferous, spreading ones) on a field scale, and this is discussed below. Legume establishment techniques are similar to those for grasses, and their seed treatment and inoculant requirements are discussed below.

When to sow

The establishment period is crucial to the success, longevity and productivity of a sward. Optimum sowing time varies greatly with local conditions and farming systems. Undersown fodders, of course, must be sown immediately after their nurse crop and the date will be dictated by the cropping cycle. In humid, temperate regions, spring and late summer are the two main sowing seasons, avoiding the possible summer drought. Spring sowings should be after the danger of serious frost and chill winds has passed. Under tropical conditions, it is usually beneficial to sow at the very start of the rains to assure as long a growing season as possible and to profit from the flush of nitrogen released by the soil on initial wetting. The shorter the rainy season, the more critical is early sowing. Where the dry season is short, pasture can be sown late in the rains. The peak of the rainy season is best avoided. The usual advice is to sow when rain is expected in the coming days - sound but difficult in practice; light rain is very beneficial but a heavy tropical storm can wash and bury the light seeds seriously.

In areas of continental climate, with severe winters and snow, but with rapid warming in spring, various techniques which take advantage of snow-melt can be used. The seedbed may be prepared and the seed sown just prior to expected snowfall; alternatively, the seedbed may be prepared in autumn, then seed sown on the snow when the spring thaw is expected. The melting snow washes enough earth over the seeds to cover them, provides moisture for establishment and growth. These methods suit the heavier seeds of legumes better than grasses.

In irrigated areas, if there is a rainy season, it may be better to sow on rain and irrigate thereafter, especially where flood irrigation is used. In much of western Asia, the spring is a season of severe winds and sandstorms (khamasin, etc.) which can sand-blast or uproot young seedlings. In areas where these winds are known to be prevalent, the season should be avoided and a light seed-rate of a cereal added to provide some protection to the germinating pasture.

Table 3. Seed weights of some grass species

Seedbed preparation - rainfed crops

A seedbed of medium tilth, as for wheat, should be aimed at under humid-temperate conditions. Where the intensity of rainfall is relatively low, finer seedbeds can be used. Too fine a tilth may lead to excessive wash, and too rough a tilth will make control of planting depth difficult and may lead to seeds and seedlings being buried. Ploughing should be done well in advance of sowing where possible, and the seedbed prepared by discing and or harrowing immediately before sowing. Rolling with a Cambridge roller, when available, is a great help in firming the seedbed. It helps push any small stones, which might hinder mowing, into the soil. The cleaner the seedbed, the greater the chances of a quick and vigorous establishment. On newly cleared land, weeds are not usually a serious problem at the establishment stage. In the majority of cases, however, hay crops will be sown on old arable land, which is certain to have a heavy weed burden. Every effort should be made to reduce this during previous cropping, but it is still likely that the hay seedlings will have to compete with many weeds. This is especially serious for tropical hay species, which have very small seeds and are often tiny, even in comparison with temperate hays, at emergence. Direct seeding of grass into stubbles that have been cleaned by herbicides is possible where agriculture is sufficiently mechanized.

Sowing

All pasture seed is small and those of the grasses are light. Seed rates are low, but the seed expensive; care in adjustment and use of sowing equipment is very necessary. The seed of temperate grasses and most legumes is free-flowing and does not present problems in passing through seed drills. Tropical grass "seed," however, is not generally free-flowing and usually consists of a large amount of inert material (empty spikelets, immature seed, but not trash or weeds) with a relatively low seed content of medium germination; for this reason it is necessary to know the pure germinating seed (PGS) or pure live seed percentage before deciding how much to sow. Cleaning the seed to a higher degree of purity is not an option, since such small differences in specific gravity are involved that wastage of seed would be high. In addition, the seed is so small that, if pure, the seed-rates would be so low as to make sowing even more difficult. Some tropical grass seed has a period of dormancy immediately after harvest, which may persist for months - Cenchrus ciliaris and Chloris gayana are among those involved. At medium altitudes in Kenya, the germination of C. gayana seed in a store at ambient temperature reached a peak at about 30 months after harvest. Seed of temperate species from reputable companies is generally of high standard and germination and purity rates are high; farm-harvested or locally-acquired seed should be tested before use. Seed rates are given under the crops. Pasture seed is expensive and, in many developing countries, difficult to obtain. Just enough to ensure a good stand should be sown; increasing the seed rate in no way compensates for poor seed-bed preparation or inadequate fertilizer at establishment, and is very costly!

Drilling

For hay, seed should be either sown in close drills or broadcast. Whatever the method used, great care must be taken in ensuring shallow sowing and even seed spread. Seed rates for pasture are far lower than those for cereals. Broadcasting the seed presupposes broadcasting the fertilizer, and that may have a marked effect in encouraging the growth of weeds during the initial stages of establishment. A cereal drill with a fluted or studded roller feed can be used for all temperate grasses and legumes, and, for some models, special auxiliary small-seed boxes with a brush feed are available, which usually broadcast the seed behind the coulters. Close-row drills are preferred. A drill has the advantage that fertilizer can be applied at the same time as the seed. Drills are perfectly satisfactory for temperate species which can be sown at depths of 1 to 3 cm. When sowing tropical species, the coulters should be raised until they hardly touch the ground, and the down-tubes pulled out to avoid the seed being too deeply covered; the merest covering is sufficient. Many tropical grasses, including buffelgrass and Rhodes grass, have awns or bristles and do not run freely, so care must be taken to avoid bridging in the hopper. The seed may be mixed with a ballast or fertilizer, but great care must be taken to avoid its separating out in the box. If seed is mixed and sown with the fertilizer, this must be done, in the field, drill-full by drill-full, to minimize separation of the two components.

The simple, animal-drawn drills and modifications to ploughs successfully used for cereals and some leguminous fodders in Asia are not suited to tropical grass seed, but could probably be used for ryegrass. Specialized grass-sowing equipment, of course, can make establishment surer, but it is expensive and difficult to justify unless large areas are involved; the Brillon seeder, which sows between two small cultipacker rollers, is a very effective tool on good, level seedbeds, but is expensive, heavy and difficult to transport.

Broadcasting

Broadcasting can be done by seed-barrow: a long, light seed box with two shafts and a single wheel, pushed by one man. There is a more modern version, tractor mounted on the hydraulic lift and PTO driven. In trials, two men taking turns on a hand barrow could easily sow 4 ha/day, and the brush-feed was quite adequate to deal with the hairiest tropical seeds. This simple and effective implement is now difficult to obtain. Hand-driven centrifugal seeders (excellent for many legumes) are not suitable for tropical grasses, and mixing seed with fertilizer and then putting it through a fertilizer spinner is totally unsatisfactory. For hand broadcasting, the seed should be mixed with a ballast of a similar specific gravity and weighed out into lots for specified areas; the field is then marked by temporary pegs so that each area is covered by one seed lot. Marking out the land and weighing the seed into lots is especially important when the labour is not experienced. Seed is very expensive, so the extra care is usually well recompensed by reducing the common mistake of too dense sowing in the first part of the field and no seed left for the last.

Early management

Despite the care taken during land preparation to reduce the weed challenge, a considerable variety of weeds will probably be present in quantity as the crop emerges. If their number is not so great as to risk shading out the crop, it is probably best to wait until the forage is high enough for a light grazing by cattle. In pure stands of grass, selective herbicides can be used to remove broad-leaved weeds; the situation is more complicated in mixtures and leguminous crops. Mowing is an alternative to grazing, but care must be taken to avoid leaving a lot of dead material on the young sward. A first cut with a forage harvester can be used to remove the cut material and use it for feed or silage.

The main aim in the first year of establishment of a perennial species is to build up a strong sward which will retain a high yield potential over the span of years foreseen, and all activities should be geared to this. As soon as the first weed control interventions have been completed, grass fields should receive a top dressing of nitrogenous fertilizer to speed up growth and develop strong individual plants. Whether early defoliations are by mowing or grazing will depend on the crop, the season, the weather and the needs of the farm production system. Both overgrazing of the young sward and poaching by heavy stocking when the land is wet should be avoided. The height of the sward at first cut or grazing will depend on the grass in question: temperate grasses should be grazed once they are about 10 cm high, and taken down to 3 to 5 cm. The individual plants of tropical grasses are mostly much larger, and grazing should begin at 25 - 30 cm high and stop at 8 - 10 cm. Short grazing periods, with the stock removed thereafter, are always necessary. Young cattle are best for grazing pastures at the establishment stage; mature stock are heavier and cause more trampling damage; sheep are more selective feeders.

Grass establishment under irrigation

The techniques are similar to those for rainfed land, but on most soils it is desirable to irrigate before final seedbed preparation, wait until the soil is workable, sow and have the seed germinate on residual moisture, and allow only a very light first irrigation. Irrigation after sowing and before emergence of the seedlings can lead to "capping" and poor stands. In areas of cold winters, it is possible to fill the profile with water immediately before the frosts and have a moist seedbed at the spring thaw. All cultivations must ensure that the land is left level and free from hollows which would allow even slight ponding with subsequent gaps or disease-prone spots (through waterlogging) for the crop. The crop is drilled (or broadcast) in the usual way and then the in-field bunds and irrigation structures are raised. Where there is adequate rainfall for establishing the seedlings, it may be preferable to rely on the rainfall for the initial stage, and irrigate thereafter.

Undersowing and nurse crops

Pasture crops are frequently sown together with cereals (nurse or cover crops). The grain is harvested and the sward develops thereafter. This saves a season and cuts out the tillage for one crop. The cereal is sown in the usual way and then the pasture seeds are immediately broadcast or cross-drilled more shallowly. The cereal at the early stages provides some shelter to the pasture seedlings but, thereafter, they suffer seriously from shading and competition for light, water and nutrients. Sowing without a cover crop usually leads to a far better establishment of the pasture. The better the cereal crop, the greater the risk of damage to the undersown grass, and if the cereal lodges, large areas of the pasture will be lost. Undersowing, which originated when cereal growing methods were less intensive and crops consequently lighter, is, nevertheless, still popular. Where legumes or grass-legume mixtures are concerned, undersowing is even less commendable.

Instead of sowing pasture in a cereal crop, it is often possible to sow the pasture seed with a light seed rate of an arable silage or grazing crop, such as oats or barley, which is harvested early in the season and gives the young sward a good chance to develop. Teff is used in this way in South Africa for establishing Eragrostis curvula.

Undersowing can be a useful technique where strong winds and sandstorms are a problem at establishment time. Grass can be undersown in row-crops; in eastern Africa, Rhodes grass and molasses grass (Melinis minutiflora) are regularly established under maize. The seed is broadcast under the maize (usually by hand) after the last weeding. The stover must be removed immediately the crop is harvested, and a slashing to remove tall weeds is desirable. The crop is not grazed until after the onset of the next rains.

Vegetative propagation of grasses

Some tropical grasses are, on occasions, propagated vegetatively, usually stoloniferous or rhizomatous species, or large, tufted or tussocky ones. It is done where seed is difficult to produce or where clonal material is used (e.g., some cultivars of Bermuda grass (Cynodon dactylon), pangolagrass (Digitaria decumbens), or elephant or Napier grass (Pennisetum purpureum). Land preparation is similar to that for sowing, but the final tilth need not be so fine. Stoloniferous or rhizomatous grasses are planted by pieces of stolon or rhizome, either hand-set at wide spacings (they soon cover the ground) or broadcast and buried by harrowing or discing into moist soil. Large tufted grasses are row planted, by splits. Clumps are dug up and the splits prepared by cutting off tops and long roots (which would in any case die) and dividing the clump into units of two or three tillers. Great care should be taken not to dry out the planting material and this work should not be done in full sunlight. Splits can be planted directly into moist soil (do not open planting holes beforehand, but only immediately at planting) but are better piled up loosely in the shade, watered and covered with sacking for a few days until new roots begin to appear. Elephant grass is planted by stem cuttings in the same way as sugar cane.

Legume establishment and seed treatment

Legumes are grown for their high quality fodder and as a soil-improving crop in rotations. Many are melliferous: white and red clover and lucerne are noted honey producers. Where large areas are to be grown, the possibility of harvesting honey should be considered. When seed is to be produced, bees are useful for pollination. Land preparation, seedbed preparation, and sowing are the same as described for grasses.

Legume seed treatment

Legume seed often requires treatment before sowing to improve the speed of germination and to assure proper infection with the appropriate root-nodule bacteria. Pasture legumes usually have a high proportion of seeds with impermeable seed coats which do not imbibe water and which, therefore, do not germinate when sown but can lie dormant in the soil until such time as the seed coats loses its water-resistance; such seeds are said to be "hard." This is an excellent character for survival in areas of unreliable rainfall, but is not desirable when rapid establishment of a fodder crop is wanted at reasonable seed-rates; such seeds require scarification.

Table 4. Seed weights of some legumes

Legumes only thrive and fix nitrogen when in symbiosis with their specific root-nodule bacteria of the genus Rhizobium. When dealing with a crop new to an area, or sowing on newly cleared land, or using fields which have not had the legume in question for some time, it is often necessary to inoculate the seed with a culture of their specific bacterium.

Identification of effective nodules in the field

Growing crops should be examined (during active growth) to establish the state of their nodulation when deciding whether or not to inoculate sowings under similar conditions. Newly-sown legumes should always be examined for nodulation, to check on the effectiveness of the inoculation process and gather information for further sowings. Effective nodulation usually takes place within a month of planting.

Seedlings should be dug up carefully (not pulled) and their roots examined. The presence of large nodules on the tap-root and the main laterals in the top two to three centimetres of soil and which show a pink coloration on cross section (due to the presence of haemoglobin) indicates successful nodulation.

Seed scarification

The purpose of scarification is to allow the entry of water into the seed through either an impermeable testa or (as in some Stylosanthes spp.) pod, thus speeding up germination. Hard-seededness is only partially a characteristic of species or cultivar and can be greatly affected by weather during seed maturation. Mechanically harvested or threshed seed is often sufficiently scarified by abrasion by the machinery. Hand-harvested and -threshed seed of tropical legumes, however, often shows much more hardness than machine-threshed seed. Three scarification methods are commonly used: mechanical, hot water and acid. Only mechanical scarification should be used in extension work unless a highly-qualified technician is present throughout the whole scarification and sowing process. The qualities of the three methods are summarized below:

Mechanical scarification can be done in small to industrial quantities, and consists of either abrasion or concussion of the seed. At a laboratory scale, a box with a removable bottom (sized to take a sheet of glass- or sand-paper) is used. The seed is spread on the abrasive paper and rubbed with a rubber pad. Medium-sized samples can be dealt with by rubbing the seed on a smooth concrete floor with rubber-soled shoes. Large samples can be scarified in rice-hullers, in concrete mixers with some sand, and in custom-built machines. Mechanical scarification is cheap and simple, treated seed can be stored for weeks, the seed can be inoculated after treatment, and the treatment does not initiate germination. Errors are unlikely unless the seed is over-enthusiastically treated, and ground to meal!

Hot water scarification can be done in small to medium quantities. The seed is immersed in hot water (often boiling) and kept for a specific time, varying according to species. The system is simple, but requires very close supervision. It has many disadvantages. Cold-water soaking is used on Trifolium alexandrinum and T. resupinatum seeds when they are to be sown into shallowly flooded fields, but there one objective of the soaking is to prevent the seed from floating. Hot-water scarification is open to operator error (cooked seed); the process initiates germination so the seed cannot be stored for even short periods, and if sown and no rain falls, there will be a lot of mortality. If sowing is not possible (e.g., heavy rain falls during the scarifying period) the seed will ferment unless constantly aerated; inoculation is difficult; some species exude a sticky mucilage on wetting and the seeds cohere (e.g., Neonotonia spp.).

Acid scarification consists of immersing the seed in concentrated acid, usually sulphuric, for a specified time. This technique is suitable for seed testing purposes and for very small quantities. It is not for use in the field!

Seed Inoculation

The purpose of inoculation is to coat the legume seed with a sufficiently high number of live rhizobia of the correct strain to provide rapid and effective nodulation in the field. Various types of inoculant are produced, but peat-based powders are recommended for field use. Great care should be taken to ensure that the inoculant is, at all times, protected from high temperatures, direct sunlight and bactericidal materials. Seed inoculation is simple, but prone to careless errors. The inoculant powder should be mixed with an aqueous solution of a sticker, 15% gum arabic is suitable and this gum is widely available in the markets of many tropical and subtropical countries. The peat powder is mixed into the sticker immediately before use; the seed is heaped on a clean concrete floor or similar surface, and the inoculant poured on and mixed thoroughly with a shovel or in a cement mixer or similar machine, but avoid mixers that have been used for seed-dressing. The quantity of liquid is determined by practice, and the aim is to cover the seed without excess of moisture. The treated seeds should immediately be spread to dry, in the shade, in a thin layer and, once dry, sown as soon as possible.

Pelleting to protect the inoculum on the seed

When seed is to be mixed with acid fertilizer, such as single superphosphate, at sowing or where there is a danger of the seed being exposed to drought and sunlight after sowing, a process of pelleting can be used to protect the inoculum. It usually involves coating the inoculated seed with either lime or rock phosphate using a sticker such as a 45% weight/volume gum arabic solution. Properly pelleted seed may be stored at room temperature for up to a month. To obtain survival in pellets, only peat-based inoculant should be used.

A common method involves using a most widely available sticker, namely commercial gum arabic, which is used as a 40% - 45% weight/volume solution. As it is slow to dissolve, it may require warming. It is best to prepare the gum arabic the day before and leave overnight. It is essential that the gum be free from preservatives. Gum arabic is in itself a nutrient, and no other nutritives (e.g., sugar) are used. The coating material must be very fine (100% to pass a #300 mesh sieve). Agricultural lime is too coarse; plasterer's whiting and very fine lime (calcium carbonate) are suitable for clovers and temperate legumes. A quantity equal to about 25% of the seed weight is needed for seeds the size of white clover; the smaller the seed, the more material required for pelleting. There are two types of Rhizobium to be considered: the type associated with clovers and most temperate legumes is a fast-growing acid-producer for which lime is used; the other type, associated with most tropical legumes, is a slow-growing alkali producer, for which rock phosphate should be used. Table 5 provides a guide to the quantities required, but these will have to be modified to suit local conditions.

Table 5. Seed sizes and coating quantities

Source: Adapted from Tropical Forage Legumes (FAO, 1988)

The inoculant powder is thoroughly mixed with the sticker immediately before use, and the adhesive is applied to the seed, sufficient to make it thoroughly sticky without becoming too wet. The coating is then applied all at once and the heap turned, or tumbled, until the pellets are formed. The seed may be sown immediately, but it is best if allowed to dry for a few hours in the shade to harden. Large pellet must be allowed to harden or their coating may fall off.